There are several industrially-important processes, typically oxidations or ammoxidations of hydrocarbons, particularly of alkenes such as propylene or isobutylene, in which there is employed a solid catalyst comprising a molybdenum compound or compounds. Many such molybdenum-containing catalysts are known, their composition normally being represented by an empirical formula which describes the active components of the catalyst as being the mixed oxides of several elements, one of which is molybdenum. The molybdenum is often present in actual fact as a molybdate or a phosphomolybdate, for example, but in any case it is typically present in a chemical combination with oxygen of one type or another. The exact nature of the molybdenum-oxygen combination is not relevant to the purposes of the present invention, however, as will be seen hereinbelow.
Examples of such molybdenum-containing catalysts, and of processes in which they are typically employed, include the following..
U.S. Pat. No. 2,941,007, to Callahan et al., describes the catalytic oxidation of olefins to produce their corresponding unsaturated aldehyde or ketone derivatives, with the oxidation of propylene or isobutylene to produce acrolein or methacrolein, respectively, being typical. This catalyst comprises what is presented empirically as being a mixture of the oxides of bismuth, phosphorus, and molybdenum, with the phosphorus not necessarily being present in all cases. The bismuth is normally bismuth molybdate or phosphomolybdate, with the molybdenum having been initially introduced into the catalyst as molybdic or phosphomolybdic acid.
U.S. Pat. No. 3,825,600, to Ohara, describes also the oxidation of propylene or isobutylene to form the corresponding unsaturated aldehyde with catalysts which are described as being a mixture of the oxides of cobalt, iron, bismuth, tungsten, molybdenum, silicon, and an alkali metal. The catalyst is characterized as being, essentially, a mixture of complex metal oxides or metallic acid salts. The molybdenum component of the catalysts is initially formulated into the catalysts as molybdate ion, typically as ammonium molybdate.
French Pat. No. 2.047.199 to Nippon Kayaku Kabushiki Kaisha also describes a mixed oxide catalyst for the oxidation of propylene to acrolein or acrylic acid which is described as being a mixture of the oxides of nickel, cobalt, iron, bismuth, and molybdenum, together with a member of the group consisting of phosphorus, arsenic, and boron and a member of the group consisting of potassium, rubidium, and cesium. Typically the molybdenum is introduced into the catalyst as a molybdate salt (e.g., ammonium molybdate), a molybdenum oxide, or molybdic acid.
Finally, although there are still a large number of other references to catalysts of this type, oxides or iron, nickel, cobalt, bismuth, phosphorus, molybdenum, and samarium or tantalum are employed as an alkene-oxidation catalyst in a process described in U.S. Pat. No. 3,639,269 to Koberstein et al., while U.S. Pat. No. 3,629,147 to Eden et al. describes a catalyst comprising oxides of manganese, tellurium, molybdenum, and phosphorus.
Generally, in all catalysts of the above-summarized types, it is possible to employ a support when and as desired, but it is typical although not essential to formulate the catalyst as a paste one component of which is fine silica gel.
As will be seen, the exact chemical state of the molybdenum in these catalysts can vary, although normally the molybdenum can be characterized as being present in chemical combination with oxygen, either initially or at any rate after the catalyst has been employed in an oxidation process in which the catalyst is exposed to reactant gases comprising molecular oxygen.
It will also be understood that catalysts of the above-described types are useful both in simple oxidations and also in ammoxidations, with the relevance of the present invention to all such uses being explained further hereinbelow.
A basic drawback of all catalysts comprising a molybdenum oxide, e.g., molybdenum trioxide or any molybdenum compound which, under the conditions of use of the catalyst, is a source of molybdenum trioxide, has been found to be that when such catalysts are employed at an elevated temperature in a reaction zone through which there is passed a gas which is subsequently to be introduced into a subsequent processing step or zone (the exact nature of which is not pertinent to the present invention), there is an appreciable contamination of the reactor effluent gas by vapors of molybdenum trioxide. As previously indicated, molybdenum trioxide as such may not have been incorporated directly into the catalyst initially, but many molybdenum compounds, and especially molybdenum oxides including molybdates and, for example, phosphomolybdates, will liberate small amounts of molybdenum trioxide vapor at elevated temperatures. Water vapor, when present, greatly increases the molybdenum trioxide volatility and aggravates the problem. While there are instances in which such molybdenum trioxide contamination of the reactor effluent gas is a matter of no particular importance, there are other situations in which the presence of this material has been found to create difficulty. One such situation, to which the present invention is specifically directed, exists in chemical reaction systems in which the effluent from the reaction stage which employs the molybdenum-containing catalyst is to be introduced into a second processing zone which comprises a second catalytic reactor operating at a temperature which is lower than that of the first reactor. As a more specific example there are the well-known reaction systems in which catalysts for the oxidation of propylene or isobutylene as previously discussed hereinabove are employed to convert the alkene feedstock into a product comprising the corresponding unsaturated aldehyde (e.g., acrolein or methacrolein), following which the reactor effluent is transferred into a second reactor in which the contained unsaturated aldehyde is further oxidized to the corresponding alkenoic acid (e.g., acrylic acid or methacrylic acid) in the presence of a second solid catalyst which it may be desired to employ at a temperature lower than that obtaining in the first reaction zone. Such catalysts are described, for example, in U.S. Pat. No. 3,567,773 to Yamaguchi et al., U.S. Pat. No. 3,644,509 to Allen, U.S. Pat. No. 3,579,574 to Van Der Meer, U.S. Pat. No. 3,541,143 to Nakano et al., and French Pat. No. 2,056,579 to Daicel.
In such situations, i.e., when the effluent of the reaction zone in which the molybdenum catalyst is employed is introduced into a subsequent processing step operating at a lower temperature, the reduction in temperature at the entrance of said subsequent processing step results in precipitation or condensation of molybdenum trioxide upon the internal surfaces of whatever apparatus is being employed in said subsequent processing step. For example and of particular importance, the precipitated molybdenum trioxide will progressively build up on the walls of connecting ducting between processing steps and/or could be transported into the interstices of the catalyst bed employed in oxidizing acrolein or methacrolein with a catalyst like those described in the above-identified acrolein-oxidation patents when, as is often the case, this second catalyst bed is operated at a temperature somewhat lower than that of the primary (alkene-oxidation) first-stage catalyst. This buildup of molybdenum trioxide in the second catalyst bed causes an increasingly serious obstruction to gas flow through that bed and also tends to cause the catalyst particles to adhere to one another, making more difficult the periodic catalyst bed replacements normally necessary in the course of plant operation. The precipitated molybdenum trioxide also fouls heat-transfer surfaces within this second catalytic reactor with resulting adverse effects upon temperature control therein.
Other processes in which the vaporization of molybdenum trioxide and resulting difficulties caused by its subsequent reprecipitation upon cooled apparatus surfaces include the catalytic oxidation of propylene or isobutylene to produce acrolein or methacrolein to be recovered as such (i.e., without a subsequent further oxidation to form the corresponding alkenoic acid) and also the ammoxidation of propylene in the presence of a molybdenum oxide-containing catalyst to form acrylonitrile. In both of these processes, even though the reactor effluent is not to be introduced into a second catalytic oxidation step, it is necessary that the reactor effluent gases be cooled, typically in surface heat exchangers, prior to recovery and product purification steps. The same problem due to molybdenum trioxide in the catalytic reactor effluent exists in these processes, the molybdenum trioxide tending to precipitate upon the cooling surfaces of the heat exchanger apparatus through which the reactor effluent gases are passed. By application of the present invention to the hot reactor effluent gas before it is introduced into such heat exchangers, the problems due to molybdenum trioxide fouling of the heat exchange surfaces are obviated.
It will be understood that, although the two-stage oxidation process for converting propylene or isobutylene to the corresponding alkenoic acid as described above is an especially clear example of a situation in which this molybdenum trioxide precipitation presents significant processing problems, there are many other process environments in which related problems may also appear. For example, there may be chemical reasons for wishing to keep the molybdenum contamination at a minimum in the effluent of any catalytic reactor in which molybdenum is a catalyst component, with what is referred to herein as the "subsequent processing zone" being any type of product recovery and/or purification scheme as distinguished from a second catalytic oxidation reactor like that of the aboveidentified acrolein-oxidation patents.
It is accordingly an object of the present invention to provide means for reducing the molybdenum trioxide content of the gaseous effluent issuing from a reactor which is operated at an elevated temperature and which contains a solid catalyst comprising molybdenum, including especially catalysts comprising compounds of molybdenum and oxygen or any other molybdenum compound which, under the conditions obtaining within the reactor, constitute a source of molybdenum trioxide.
It is another object to provide a means for eliminating difficulties caused by molybdenum trioxide precipitation when the gaseous effluent of a molybdenum-catalyzed reaction system as just described is introduced into a subsequent processing step operating at a temperature which is lower than that of said molybdenum-catalyzed reaction.
It is a specific object to provide a method for eliminating or reducing process difficulties caused by precipitation of molybdenum trioxide in the second-stage reaction step of a two-step reaction system comprising a primary reaction of a lower alkene such as propylene or isobutylene to form a gaseous product comprising the corresponding unsaturated aldehyde such as acrolein or methacrolein using a molybdenum-containing catalyst, the gaseous effluents of this primary reaction step then being introduced into a second reaction step which operates at a temperature lower than that of the primary reaction and within which the unsaturated aldehyde is further oxidized catalytically to the corresponding alkenoic acid such as acrylic acid or methacrylic acid.
Other objects of the invention will be apparent from the following detailed description and claims.